Anyone know of a application that will decode a SIP VoIP conversa tion?

["Compton, Rich" <RCompton () chartercom com>]

I have some colleges at my place of business that are doing SIP over the
Internet without encryption and I'd like to be able to show them how
insecure this is.  I'd like to be able to provide them with some proof by
sniffing their conversation and replaying it as a .wav file or something.
They are using Nortel clients.  I have been unable to find anything in the
usual places like packet storm or Google.  Does anyone have any ideas where
I could find something like this.  I've captured some packets in ethereal
and it does decode lots of information but not exactly the shock value that
I'm looking for.
 
Any help would be appreciated!
 
-Rich Compton